drm/amd/display: stop using switch for different CS revisions

Clock sources currently have support for asic specific
function pointers. But actual separation into functions
was never performed, leaving us with giant functions that
rely on switch.

This change creates separate functions, removing switch use.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>